It is often necessary to transmit data and/or power across a rotary interface, such as the interface between a rotating member, such as a rotor, and a stationary member, such as a stator. For example, computed tomography (CT) scanners as well as other applications require data transmission across a rotary interface. In order to facilitate data transmission across the rotary interface, a slip ring is generally employed having a rotating element that rotates with the rotor and a stationary element affixed to the stator.
Initially, slip rings were developed to support electrical communication between a rotor and a stator. As data rates increased, however, electrical transmission of the data became impractical. As such, slip rings were then developed to support optical communications across the rotary interface, such as between a rotor and a stator. Optical communication could transmit data at much higher rates than prior electrical communication techniques.
Fiber optic rotary joints are generally categorized as either an on-axis rotary joint in which the optical fibers that will communicate lie along the axis of rotation or an off-axis rotary joint in which the optical fibers do not lie along the axis of rotation, typically because the axis of rotation is inaccessible. In conjunction with fiber optic rotary joints that support optical communications between the rotor and stator of a CT scanner, for example, the axis of rotation extends centrally through the bore or tube in which the patient is disposed. Thus, optical fibers and other optical elements that support communication between the rotor and stator cannot practically be disposed along the axis of rotation without disadvantageously interfering with the already limited space in which the patient lies.
Off-axis rotary joints generally include channel waveguides to direct the optical signal. In this regard, off-axis rotary joints generally include multiple optical sources, driven by one or more lasers, and multiple receivers in communication with respective channel waveguides. The multiple optical sources may be disposed circumferentially about either the rotor or the stator, while the receivers are disposed circumferentially about the other one of the rotor or the stator. For example, multiple optical sources may be disposed circumferentially about the rotor, while multiple receivers are disposed circumferentially about the stator, thereby supporting optical communications from the rotor to the stator.
In operation, each of the optical sources transmits the same optical signals. These optical signals are received by one or more of the receivers, depending upon the angular position of the rotor relative to the stator. While generally effective for permitting optical communication between a rotor and a stator, conventional off-axis rotary joints that employ channel waveguides do suffer from several shortcomings, especially at relatively high data rates.
As a result of the construction of a conventional off-axis rotary joint, the optical signals generally propagate along paths between the respective optical source and the respective receiver that have different lengths, thereby introducing varied time delays in the propagation of the optical signals. By way of example, a receiver of a conventional off-axis rotary joint commonly receives the same data from each of two adjacent optical sources. However, the optical signals emitted by the two optical sources travel different distances to reach the receiver and, as such, are received at somewhat different times. Accordingly, the pulse width of the optical signal is effectively broadened. To support communication at the high data rates that are desired, conventional off-axis rotary joints may need to be redesigned to have less spacing between the optical sources and the receivers and may eventually be unable to be further redesigned to support even higher data rates.
By way of example, one conventional fiber optic rotary joint has 16 optical sources spaced evenly in a circumferential manner about a slip ring having a diameter of 46 inches. Thus, the spacing ΔL between adjacent optical sources is ΔL=π*d/16=9 inches (0.229 m). Accordingly, the time delay introduced by the separation of adjacent optical sources is Δt=ΔL/c=0.76 nsec. For a fiber optic rotary joint designed to support data transmitted at 1.25 Gbit/sec, the pulse width of each bit of data is Δw=1/1.25 GHz=0.8 nsec. As such, for a receiver that receives the same optical signals from two adjacent optical sources, the time delay introduced by the spacing between the adjacent optical sources effectively lengthens the pulse width from 0.8 nsec to 1.56 nsec, that is, 0.8 nsec+0.76 nsec. As such, it will be difficult for the fiber optic rotary joint of this example to support error-free data transmission at 1.25 Gbit/sec, let alone to support communication at the even higher data rates that are desired.
In order to support higher data rates, a conventional fiber optic rotary joint may be redesigned to effectively reduce the spacing between adjacent optical sources, such as to within four inches (10.1 cm), which will introduce a time delay of 0.34 nsec between the optical signals transmitted by adjacent optical sources. Even with the redesign of the fiber optic rotary joint, the optimization of the detection electronic circuitry and careful alignment of the channel waveguides, a conventional rotary joint has difficulty supporting data rates greater than 1.25 Gbit/sec.
Conventional off-axis fiber optic rotary joints may also have additional shortcomings. In this regard, conventional off-axis rotary joints have relatively high losses. As such, conventional off-axis rotary joints require optical sources that operate at higher power levels to produce optical signals having more power, thereby creating issues relating to heat generation and disposal and requiring electronic driver circuitry having greater complexity. Additionally, conventional off-axis rotary joints having a plurality of channel waveguides also generally have a plurality of optical fibers for directing the optical signals from the channel waveguides to a photodiode. The plurality of optical fibers are bundled together and coupled to a photodiode via a lens assembly. As the data rate increases, however, a photodiode having a smaller active area is required. The increased ratio of the fiber diameter to photodiode area makes it more difficult to focus multiple optical signals onto the relatively small active area.
While conventional off-axis rotary joints support optical communications between a rotor and a stator, it would be desirable to provide an improved off-axis rotary joint. In particular, it would be advantageous to provide an off-axis rotary joint capable of supporting optical transmission between a rotor and a stator at relatively large data rates, such as 1.25 Gbit/sec and greater.